Deciding on using rafters or trusses for a shed roof is a major decision. It may seem simple to just put up rafters until you realize all the angled cuts that must be figured.
Trusses may seem to be a simple answer until you start wondering how heavy they will be and how you will get them to the top of your walls. You’re going to have a lot of questions. Below are the answers to some of those likely questions and the pros and cons of both trusses and rafters.
What’s the Difference Between Rafters and Trusses?
Rafters are built from scratch from lumber by the builder. The common board widths used to create the roof slope are 2x8s, 2x10s or 2x12s. Rafters are usually cut on the ground, then hoisted to the roof to be connected to the roof ridge board.
The ridge board runs along the length of the roof and is usually the same width as the rafters. The ceiling joists are joined to the bottoms of the rafters to form the frame of the ceiling. Shorter boards, known as collar ties, are usually fastened horizontally between paired rafters to add support to the rafters.
If your shed plans include roofing insulation, it may be installed between these rafters or between the ceiling joists.
Trusses are pre-built in a factory, using a triangular webbing configuration of boards to tie the outer walls together and support the roof. While trusses are usually constructed with 2x4s instead of wider boards, they do incorporate more boards in the design to add stability and strength. They are generally considered to be stronger than rafters. Trusses also include bottom joists.
The Advantages and Disadvantages of Trusses
Advantages. Since they are built in a factory, the lumber cuts are automated. This results in cuts that are extremely precise. The specifications provided by the customer are loaded into custom software, and the truss materials are measured digitally and cut. The process ensures that each truss meets the specifications.
Much time can be saved using roof trusses. The actual installation only takes about a day on a 2,500 square foot home, so you can see how little time your shed trusses will take to install. On the same home, rafters may take up to a week to install, depending on the number of people involved in the construction.
This time savings will enable you to get the roof sheathing put on so that the shed is protected from the weather. Anybody trying to construct something in a climate that often has intermittent rain showers will especially benefit from this.
The lead time on a truss system is usually three to four weeks, but could be longer when demand is high because of a busy construction season. They should be ordered well ahead of time.
Trusses are stronger than you may think. Even though the lumber used is not as wide as when using rafters, the extra material and webbing effect of the construction makes for great structural strength.
Truss spans can be used to span spaces up to 60 feet, while rafters are recommended for spans up to 30 feet. If your shed is likely to be covered with snow in the winter, this distance for rafters decreases.
Disadvantages. Prefabricated trusses are large and heavy, and will likely be delivered on a semi-truck. For your shed, a few friends helping for an afternoon should suffice to unload and install them. Their weight does make them harder to handle than raw lumber, however.
The webbing structure of trusses means that an open attic is not possible. If you plan to store items in your shed’s attic, this should be a consideration.
If your shed roof should suffer damage from a storm or fallen limb, it will be harder to repair when trusses are used than with rafters. You may have to replace a whole truss rather than replacing a piece or two, and this can be a very involved process. If your wondering what the best type of roof is read my article The 7 Most Popular & Practical Shed Roof Designs.
The Advantages and Disadvantages of Using Roof Rafters
Advantages. Using rafters creates an open attic, making it useful for storage. You can even leave the ceiling joists open in more than one place instead of creating a solid ceiling with a hatch to make access easier to put items up there.
Another option is to use loose plywood sheeting which can be moved as needed to place items. If you’re storing boxes of things that were previously stored in the house, many sizes can be placed right on the open joists.
Since rafters come in raw lumber, they can be brought to your building site in a pickup truck. Instead of waiting for trusses to be delivered you can buy the lumber, take it home and immediately start building. You also won’t need help to raise the rafters to the top of your walls.
Disadvantages. The cost is more with rafters, especially with labor costs factored in. Even if you are doing the work yourself, your time is valuable and should be taken into consideration when figuring costs.
Building rafters and installing them takes much more time than using prefabricated trusses. A pop-up rainstorm can easily interrupt your work, leaving you no choice but to stretch tarps over your shed for protection and waiting until everything is sufficiently dry to take up where you left off.
However, since building a shed roof with rafters is not as time-consuming as using them on a larger building such as a house, you may take advantage of a weather forecast for a dry week.
Building rafters takes quite a bit of skill. Properly measuring the angles for the various kinds of pieces is a brain exercise in itself. Your top needs to be cut at the proper angle to meet the roof ridge snugly, while the bottom end usually takes two cuts to form the tail cut.
Likewise, the collar ties and ceiling joists need angled cuts as well. It’s almost impossible even for an experienced carpenter to get all the rafters and other parts cut exactly the same and to exact specifications.
If you’re not experienced in measuring and cutting angles, this is not the kind of project you want to use to learn. If using rafters, it would be a good idea to allow for mistakes and order more lumber than you think you’ll need. It’s much better than having to go on an emergency trip back to the lumber yard for replacements.
Are Trusses More Work to Install?
Actually, trusses are less work to install than rafters. Rafters have to be measured, sawed and installed piece by piece in place on top of the roof. Properly measured trusses have only to be hauled to the top of the shed, then attached. If the trusses have been ordered to the right measurements, the process is much easier and less time-consuming than building rafters.
Truss packages come with detailed instructions as to fastening and spacing. If your package includes trusses of different sizes, such as those used in gables, they will be marked to indicate placement. Once you have solved the process of getting them up to the roof, the process goes quickly.
Do Trusses Cost More Than Using Roof Rafters?
You may think that just buying the raw lumber and building rafters yourself will cost less than a prefabricated truss system. However, when comparing the cost of a prefabricated truss package with material and labor costs of building rafters on-site, trusses usually cost 30-50 percent less, even with the larger shipping costs.
How Do I Measure for a Truss?
Before you start measuring, you’ll need to decide on the pitch of your roof. When planning your shed, you should have decided on the type of roofing material you want to use. This comes into play when figuring roof pitch. Common roof pitches are between 4/12 and 9/12.
This means that the roof rises either four or nine inches for every 12 inches of length. This number gives you the highest point of your roof. Some pitches are not suitable for all roofing materials.
Besides your roofing material, the climate in your area is also a consideration, as areas prone to snow often need steeper pitched roofs to enable snow to slide off. Talk to your roofing supplier to see what pitch he suggests for your roofing materials and local weather conditions.
You’ll need a helper to measure for a truss. A ladder should be placed at each end of your shed and pull a tape measure from one end of the top plate to the other to get the length measurement. Repeat this to get the width of the shed.
To figure the number of trusses you’ll need, divide the length of the roof plate by two. Trusses are generally placed every 24 inches.
Lastly, you’ll figure the length of your truss rafters. A common formula for doing this is to add twice the roof span to twice the rise to the highest point. The square root of your results will give you the length of rafters to be used in the span.
Another 12 to 18 inches should be added for overhang. If you aren’t sure about your results, there are online calculators. Your truss supplier should also be able to help you with this figure as long as you have your shed width and pitch height numbers.
Can I Build My Own Roof Trusses?
A person certainly can build their own roof trusses. If you have sufficient help to haul them to the top of the walls, they can be pre-built on the ground. Be aware that you’ll have the same problems as in building rafters, namely measuring and cutting angles precisely and consistently.
You probably won’t get the same results in terms of fit as you would in getting prefabricated trusses, as they are designed and cut using machines run by software to ensure consistency and accuracy. It can be done but really negates some of the advantages of using trusses.
How to Install Roof Trusses?
You definitely will need some help to raise the trusses. Be sure to ask your truss manufacturer if information on bracing and as well as lifting and hoisting procedures will be included with the truss package. If not, he probably can furnish you with these. Because trusses are heavy, it’s vital to handle them properly to avoid accidents.
Begin measuring for the trusses from the back of the building. The first mark should be at 23-1/4 inches, while the rest are 24 inches apart. This will keep the trusses on 24-inch centers. The two end trusses will be flush with the outside framing of the wall ends.
Ideally, you’ll have three helpers to raise the trusses, two on the ground and two on the wall or ladders. Bring each truss inside the shed, then put one end on top of the wall and then the other. It may be easier to do this if you hold them upside down and then turn them.
You can nail 2×6 boards to the wall to hold the trusses in place while fastening. The temporary bracing will be removed once the trusses are fastened, so don’t sink the nail on this bracing.
Ideally, your gable trusses should be sheathed on the ground. If you’re running short of lifting help, it can be done later. Once you get the gable truss in place, make sure it is centered. Use 16 penny nails to go through the bottom chord of the truss through the ceiling backer every 16 inches.
You’ll also need to install temporary bracing to the top chords of the trusses to make sure they stay equally spaced on top as well as the bottom. While installing each truss, have a helper make sure they stay in plumb. A good way to do this is to nail a temporary brace from the bottom chord of the fourth truss to the top of the gable truss to keep it in place. Do this on both gable ends.
Once your trusses are all nailed in place, your roof sheathing will help hold it there and should be installed as soon as possible afterward. Remove your temporary braces as you work installing the sheathing, one truss at a time. Your truss package should come with a booklet that explains the location and method of installing permanent bracing for the trusses.
The use of trusses certainly comes with its own set of complications, mostly in the transport and raising. However, once you’ve rounded up a few friends to help, it shouldn’t take up more than a day of their time for your shed, and probably less.
When contrasted with the skill needed to figure measurements and angles, and the process of precisely cutting the angles of rafters, most people will gravitate toward the truss solution. You may find my article How to Choose the Best Roofing Material for a Shed helpful when choosing trusses over rafters.
Trusses have become so popular that it’s getting hard to find experienced carpenters with the skills to construct rafters, as they keep retiring from the trade and not enough younger carpenters are willing to carry on the craft. When building a shed, they certainly are an option worth considering.
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